The present invention pertains to modulation and more particularly to digital phase modulation and modulators.
Typical quadrature modulators that yield bandwidth limited constant envelope signals are implemented utilizing analog components. Other quadrature modulators are fabricated using a hybrid structure of analog and digital components. Such modulators exhibit the following kinds of problems: first, significant tuning problems; second, signal drift with temperature and aging of components; and third many components are required for the implementation of analog and hybrid modulators; and fourth requires a large number of components to achieve low out-of-band energy transmission.
Constant envelope signals are important in communication systems where the communication channels have non-linearities. An example of such systems with non-linearities is a satellite power amplifier driven in its non-linear region for efficiency reasons. Pulse shaping and phase modulation circuitry are useful in transmitters for communications systems which require such constant envelope signals. Fully digital programmable shaping and phase modulation circuitry is particularly useful for such phase modulation schemes as M-ary phase shift keying (PSK), trellis coded modulation (TCM), and continuous phase modulation (CPM). Pulse shaping is of critical importance since it reduces out-of-band spectral power that can affect the link performance in adjacent channels.
Rudimentary full digital quadrature modulators have been designed. However, such fully digital quadrature modulators employ digital multipliers. Digital multipliers inherently reduce the operating speed of such circuitry and therefore reduce the throughput of such digital modulators.
What is needed is a high speed fully digital pulse shaping and phase modulator for reducing out-of-band spectral energy.